Different applications related to dc-ac conversion such as motor drives and grid connected applications, including UPS systems, can be fed by fuel-cells, photovoltaic (PV) panels, batteries or other low-voltage (LV) dc sources. Because of this it is of interest to use low-voltage metal oxide semiconductor field effect transistors (MOSFETs). Quite often the dc input is not sufficient to meet the ac side requirements. Therefore, a step-up converter in series with an inverter is often employed.
In many cases, high-voltage gain converters, such as voltage multiplier or isolated solutions, are mandatory and this highly affects the efficiency of the system. This is particularly true for fuel-cell applications, as the output voltage is very low. On the other hand, the inversion stage typically uses high-voltage rating semiconductor devices, MOSFETs or IGBTs, depending on the voltage and the power levels of the targeted application. Such utilization of high-voltage semiconductor devices leads to significant switching and conduction losses, resulting in a deteriorated system efficiency.
Connecting low-voltage dc sources to ac systems usually requires an additional step-up conversion stage before inversion is made to ac. Such additional stage brings extra complexity, conversion losses, and cost to the entire power conversion system. Furthermore, high-voltage Insulated-Gate Bipolar Transistors (IGBTs) are often required in the inverter, which introduces higher switching and conduction losses. In other words, the utilization of low-voltage MOSFETs and benefiting from their low ON-state resistance is quite limited. Moreover, there are often output filter requirements that are quite challenging and the use of output filters significantly contributes in system losses.
There is therefore a need for an improvement regarding one or more of the above-mentioned issues.